Image non-uniformity has long been a difficult problem for most digital marking processes. Streaks are one-dimensional image defects that generally run parallel to the process direction in a printed image. They are typically caused by the undesirable non-uniform response of some components in a marking engine. Defects in the subsystems of a xerographic printer, an inkjet printer, or similar image forming system may give rise to visible streaks in a printed image. For example, photoreceptor scratches, contamination of the charger wire, non-uniform LED imager and Raster Output Scanner (ROS) spot size variations are examples of subsystem problems giving rise to rendered image streaking in a xerographic marking engine. Bands are also one-dimensional image defects that generally run perpendicular to the process direction in a printed image. They are typically caused by time-varying performance of a marking subsystem such as nonuniform velocity of the photoreceptor drive out-of-roundness of development rolls, and wobble of the ROS polygon mirror. In a uniform patch of gray, streaks and bands may appear as a variation in the gray level. In general, “gray” refers to the optical density or area coverage value of any single color separation layer, whether the toner is black, cyan, magenta, yellow or some other color. FIG. 1 is an exaggerated illustration of streaking as it would appear in a printed document.
Another way to describe what causes image streaking is that the marking tone reproduction curve (TRC) of the printer is a function of position in the cross-process direction. For example, a light streak gives lower print densities as a function of input gray level when printing over the streak than when printing away from the streak.
Most of the existing methods to mitigate streaks are “passive”; i.e., they do not employ sensing and compensating for non-uniformities, but rather, they require using subsystem components that operate in a very uniform manner (e.g., a more accurate optical system for the ROS). As a result, manufacturing and maintenance costs increase dramatically in order to cope with more stringent image quality requirements. One method of reducing streaks is to design and manufacture the critical parameters of marking engine subsystems to tighter specifications. Such precision manufacturing, however, will often prove to be cost prohibitive.
It has been proposed to modify the digital input image with spatially varying digital image processing TRCs, to compensate for streaks. Spatially dependent TRCs are provided that compensate for a non-uniform engine response so that each gray level throughout target space is printed at desired values. Processes have been implemented that accurately measure engine response curves “ERCs.” Iterative methods such as feedback control shown in FIG. 3 (labeled as prior art), have been used to compute a suitable compensation for overcoming inaccuracies, adjusting color and reducing noise. Prior image processing solutions, however, presented fundamental concepts of streak compensation by providing detailed measurement and iteration methods, but did not develop details for an optimal method of selecting a set of TRCs that is practical and cost effective for common printing image path architectures.
An TRC may be measured by printing patches of different bitmap area coverage. In some digital image processing applications, the reflectivity of a patch of gray is measured with a toner area coverage sensor. This manner of operation over a toner area coverage sensor is described in U.S. Pat. No. 4,553,033. Toner area coverage sensors are typically designed with an illumination beam much larger than the halftone screen dimension. This large beam, however, does not provide the resolution for the toner area coverage sensor to be useful as a sensor for the narrow streaks that may occur for poorly performing subsystems.
U.S. Pat. No. 6,021,285 describes an image quality control apparatus that controls the quality of xerographic images formed by a xerographic imaging system onto a recording medium. A sensor provides signals to the xerographic imaging system for quality control malfunctions. The image quality control apparatus includes a controller device in communication with the xerographic imaging system. The controller includes a data collection device, a determining device and an input generating device. The data collection device collects and processes sensor data received from the at least one sensor while the at least one sensor is operative. The determining device determines whether the at least one sensor malfunctions. The input generating device generates a controller input signal by using the sensor data collected by the data collection device and provides the controller input signal to the xerographic imaging system to control quality of the xerographic images when the determining device determines that the at least one sensor has malfunctioned.
U.S. Pat. No. 5,963,244 describes the recreation of a TRC by providing a look up table. The look up table (LUT) incorporates a covariance matrix of elements containing n tone reproduction samples. A matrix multiplier responds to sensed developed patch samples and to the LUT to reproduce a complete TRC. A controller reacts to the reproduced TRC to adjust machine quality.
U.S. Pat. No. 6,636,628 describes an iteratively clustered interpolation (ICI) algorithm for use with irregularly sampled color data to develop a structured inverse. The algorithm is proposed to improve device independent color across devices, such as, for example, printers, scanners and displays.
U.S. Pat. No. 5,749,020 describes fundamental machine functions such as the TRC that need to be divided into regions of smaller units so that each unit can be interrelated to some aspects of the internal machine process.
U.S. patent application Ser. No. 09/738,573 by Klassen et al discloses a method for compensating for streaks by introducing a separate TRC for each pixel column in the process direction, which requires ample computing resources and memory. Using the Klassen invention, a compensation pattern is printed and then scanned to first measure the ideal TRC, and then streaks are detected and measured. The TRCs for the pixel columns associated with the streak are then modified to compensate for the streak.
The foregoing patent references are commonly owned by the assignee of the present invention and are incorporated herein by reference in their entirety. What is still apparently needed in the art are methods and system that can address shortcomings in achieving print uniformity, e.g., by reducing image streaks, while requiring less system resources and time. Given the shortcomings in the art for providing cost effective image streak compensation systems and methods, the present inventors now provide a solution for effectively and efficiently overcoming image defects.